dnd: Add a drag-cancelled signal

This lets us start the workspace zoom out animation right when the
snapback starts, rather than waiting for the snapback to finish.

https://bugzilla.gnome.org/show_bug.cgi?id=643786

js/ui/overview.js

@@ -481,6 +481,10 @@ Overview.prototype = {
         this.emit('window-drag-begin');
     },
 
+    cancelledWindowDrag: function(source) {
+        this.emit('window-drag-cancelled');
+    },
+
     endWindowDrag: function(source) {
         this.emit('window-drag-end');
     },